instances of the same object : any obvious way to "mark" them as such in the outliner?
Since there is still no obvious way to Mark Instances &amp; having to manually identify them is a bit messy. Maybe it would be good for future updates to consider adding this somehow.
My suggestion. Look at the way Modo 3d from the Foundry does this. It's simple and effective.
The Instance in the Item List (Outliner), is coloured pale pink and has it's name shown in italics. However, if you don't have the outliner visible, you still need a way to discern the Instances.
In the 3d Viewport of Modo, the Instances are shown as wireframes of the same pink shade when unselected. When they are selected, they conforms to normal object shading, the same as the source object.